This invention relates to doubly fed induction generators and more particularly to the control of the torque and power factor of such generators and the synchronization thereof to the utility grid.
An induction machine with a wound rotor and slip rings is one of the possible generator configurations used in a wind power generation system. A similar machine is also used as a variable speed drive for some drives applications in the cement and minerals industries. Such a machine is also called a doubly fed or cascade machine because the electrical power is applied at the stator as well as at the rotor terminals.
In either of the applications described above, the stator winding is connected to the utility grid and a three phase inverter is connected to the rotor windings through slip rings. An electronic controller is used to control the on-off states of the inverter switches to thereby control the torque of the machine. In the wind power generator, the electronic torque controller can be used to control the desired amount of generated electrical power while in the variable speed drive, the torque controller is used to control the speed of the motor. In addition, the same torque controller can also be used to control the power factor of the generator or the drive system to a desired level which is normally equal to 1. In the generator application, the torque controller also performs the task of xe2x80x9csynchronizationxe2x80x9d so that the generator system can be connected to or disconnected from the utility grid without any disruptive voltage and current transients.
A method to control the torque of a doubly fed machine is described in the published PCT patent application WO 99/07996. Such a torque control technique is based on a well known and published method called Field Oriented Control or Vector Control of induction machines.
U.S. Pat. No. 4,918,367 (xe2x80x9cthe ""367 Patentxe2x80x9d) which is assigned to an entity related to the assignee of the present invention describes a technique called Direct Torque Control (DTC) to control the torque of a squirrel cage induction machine whose stator is fed by a three-phase inverter. The DTC technique controls the torque of the squirrel cage induction machine by directly controlling the state of the inverter switches based on the estimated value of the motor torque and motor flux. The motor model (in software) uses the measured values of the motor voltages and currents and estimates the torque and the stator flux. FIG. 1 shows the principle of DTC of an induction machine 10 which is not a doubly fed induction machine.
The DTC block 12 receives torque and flux commands and the motor model 14 supplies the estimated motor torque and estimated motor flux as feed back signals to block 12. Block 12 includes flux comparator 16 and torque comparator 18 to determine a flux and torque error, respectively, as the difference between the estimated motor flux and estimated motor torque and the associated command. Based on the torque and flux errors, the DTC block 12 chooses the one of the possible eight inverter voltage vectors (six non zero and two zero), which tend to xe2x80x9cpullxe2x80x9d the stator flux vector "psgr"s, to control the xe2x80x9cmovementxe2x80x9d of the stator flux with respect to the rotor flux. Since the torque is proportional to the area of the triangle formed by the stator and rotor flux vectors "psgr"s and "psgr"r, the torque can be increased or decreased by advancing or retarding the stator flux vector "psgr"s with respect to the rotor flux vector "psgr"r by choosing appropriate voltage vectors.
The xe2x80x9cvoltage vector selectorxe2x80x9d 19 of DTC block 12 selects one of the eight possible inverter voltage vectors based on the outputs of the flux and torque comparators 16, 18 and the present sector location of the stator flux vector "psgr"s as determined by sector selector 17. The logical outputs SA, SB, SC of voltage vector selector 19 represent the desired (on or off) states of the inverter switches 11. The inverter 11 has upper and lower switches for each of the three phases A, B and C. When output SA=1 or SB=1 or SC=1, the phase A or phase B or phase C upper inverter switch is on and lower inverter switch is off. When output SA=0 or SB=0 or SC=0, the phase A or phase B or phase C upper inverter switch is off and lower inverter switch is on.
It is desirable to use the principle of DTC to control the torque of a doubly fed induction machine. More particularly, it is desirable to use the principle of DTC to control the torque of such a machine used in a wind power generation system. Further it is also desirable to use the principle of DTC to control the power factor at the stator terminals of the doubly fed machine to a desired level and also to perform synchronization of the wind power generation system that uses a doubly fed machine. The controller of the present invention allows the principle of DTC to be applied in all of the foregoing aspects to a system that uses a doubly fed machine and more particularly to a wind power generation system that uses such a machine.
A method for controlling the torque and power factor of a doubly fed machine using direct torque control. The method has the following steps:
(a) calculating the estimated torque of the machine;
(b) determining a torque error from the estimated torque and a reference torque;
(c) calculating the desired rotor flux command xcexa8r_ref;
(d) calculating the actual rotor flux xcexa8r;
(e) converting the actual rotor flux from the stator reference frame to the rotor reference frame by multiplying xcexa8r by exe2x88x92jxcex8m;
(f) determining a flux error from the desired rotor flux command and the actual rotor flux converted to the rotor reference frame; and
(g) selecting an inverter voltage vector from the torque error and the flux error.
A method for synchronizing a doubly fed machine that has an induced stator voltage using direct torque control to an electrical grid having a grid voltage by controlling the tangential motion and radial length of a flux vector for the rotor of the machine. The method has the steps of:
(a) determining the stator flux vector from the induced stator voltage;
(b) determining the grid flux vector from the grid voltage;
(c) calculating the angular error between the stator flux vector and the grid flux vector;
(d) comparing the angular error to zero to determine an error signal for controlling the rotor flux vector tangential motion;
(e) calculating the desired rotor flux command xcexa8r_ref;
(f) calculating the actual rotor flux xcexa8r;
(g) converting the actual rotor flux from the stator reference frame to the rotor reference frame by multiplying xcexa8r by exe2x88x92jxcex8m;
(h) determining a flux error from the desired rotor flux command and the actual rotor flux converted to the rotor reference frame, the flux error for controlling the rotor flux vector radial length; and
(i) selecting an inverter voltage vector from the torque error and the flux error.
A method for synchronizing a doubly fed machine that has an induced stator flux vector using direct torque control to an electrical grid having a grid flux vector by controlling the tangential motion and radial length of a rotor flux vector for the machine. The method has the steps of:
(a) calculating the angular error between the stator flux vector and the grid flux vector;
(b) comparing the angular error to zero to determine an error signal for controlling the rotor flux vector tangential motion;
(c) calculating the desired rotor flux command xcexa8r_ref;
(d) calculating the actual rotor flux;
(e) converting the actual rotor flux from the stator reference frame to the rotor reference frame by multiplying xcexa8r by exe2x88x92xcex8m;
(f) determining a flux error from the desired rotor flux command and the actual rotor flux converted to the rotor reference frame, the flux error for controlling the rotor flux vector radial length; and
(g) selecting an inverter voltage vector from the torque error and the flux error.
In a controller for controlling the torque and power factor of a doubly fed machine using direct torque control. A method that has the steps of:
selecting either a first operating mode of the controller for controlling the torque and power factor of the machine or a second operating mode of the controller for synchronizing the machine to an electrical grid;
the method comprising when the first operating mode is selected the further steps of:
calculating the estimated torque of the machine;
determining a torque error from the estimated torque and a reference torque;
calculating the desired rotor flux command xcexa8r_ref;
calculating the actual rotor flux xcexa8r;
converting the actual rotor flux from the stator reference frame to the rotor reference frame by multiplying xcexa8r by exe2x88x92jxcex8m;
determining a flux error from the desired rotor flux command and the actual rotor flux converted to the rotor reference frame; and
selecting an inverter voltage vector from the torque error and the flux error.